1 Technical Field of the Invention
The present invention relates to an exhaust gas cleaning apparatus for cleaning the exhaust gas from a lean burn internal combustion engine.
2 Description of the Related Art
In a lean burn internal combustion engine, such as a diesel engine or a lean burn gasoline engine, combustion is made at an air/fuel ratio higher than the stoichiometric air/fuel ratio. Due to the excessive air in the air/fuel mixture, it is ineffective to reduce the nitrogen oxides (NOx) contained in the exhaust gas by means of a three-way catalyst. Accordingly, a Lean NOx Trap (LNT) is used instead to reduce the NOx contained in exhaust gas. The LNT is configured to absorb the NOx when the exhaust gas is lean and reduce the absorbed NOx to harmless nitrogen (N2) and water (H2O) when the exhaust gas turns to rich.
To make the normally lean exhaust gas turn to rich, one approach is to increase the quantities of fuel injected into cylinders of the engine, thereby making the air/fuel mixture in the cylinders rich. However, particularly when the engine is a diesel engine, for making the air/fuel mixture rich, it is necessary to inject a large quantity of fuel into the cylinders of the engine. Accordingly, with the large quantity of fuel, it is difficult to suppress the amount of smoke generated by the combustion of the air/fuel mixture in the cylinders.
Another approach is to directly add fuel to the exhaust gas. More specifically, according to this approach, a fuel injector is mounted on an exhaust pipe of the engine to inject fuel into the exhaust pipe; the injected fuel is then added to the exhaust gas flowing through the exhaust pipe, making it rich; the richened exhaust gas further flows to the LNT located downstream of the fuel injector.
As a related prior art, Japanese Patent No. 2692380, an English equivalent of which is U.S. Pat. No. 5,483,795 discloses an exhaust purification device which can detect when the NOx absorbed in a NOx absorber is completely released from the NOx absorber. The device can also detect the deterioration degree of the NOx absorber based on the time required for the NOx absorbed in the NOx absorber to be completely released from the NOx absorber.
To more effectively reduce the NOx contained in the exhaust gas, Exhaust Gas Recirculation (EGR) can be used in combination with the LNT. EGR is a NOx reduction technique which works by recirculating a portion of the exhaust gas back to the cylinders of the engine.
In particular, when the engine is equipped with a turbocharger, two EGR systems can be arranged around the engine. The first EGR system is a high-pressure EGR which returns the exhaust gas from an exhaust pipe located between an exhaust port and an exhaust gas turbine to an intake pipe of the engine; the second one is a low-pressure EGR which returns the exhaust gas from an exhaust pipe located downstream of the exhaust gas turbine to the intake pipe.
With the two EGR systems, it is possible to recirculate a sufficient quantity of the exhaust gas at a high load. More specifically, at a high-load, it is necessary to feed an increased quantity of the exhaust gas to the exhaust gas turbine to increase the intake air pressure of the engine; accordingly, only a decreased quantity of the exhaust gas can be returned to the intake pipe by the high-pressure EGR. However, since the exhaust gas fed to the exhaust gas turbine can also be returned to the intake pipe by the low-pressure EGR, a sufficient total quantity of the exhaust gas can be returned to the intake pipe at the high-load.
By returning the exhaust gas through the low-pressure EGR, the combustion temperature in the cylinders of the engine can be lowered, thereby decreasing the amount of NOx contained in the exhaust gas. Moreover, the NOx reduction performance of a LNT generally drops with increase in the engine load. Therefore, to reliably reduce the NOx contained in the exhaust gas at a high load, it is preferable to use the LNT in combination with the low-pressure EGR.
However, up to now, no technique or approach has been developed to suitably control a lean burn internal combustion engine where a LNT and a low-pressure EGR are used in combination and where both the EGR and a fuel injector, which injects fuel into the exhaust pipe for enabling the LNT to reduce the NOx, also need to be suitably controlled. Moreover, there are several problems to be considered in controlling both the low-pressure EGR and the fuel injector.
For example, when the fuel injector injects fuel into the exhaust pipe for enabling the LNT to reduce the NOx while the low-pressure EGR is in operation, a portion of the injected fuel will be returned to the intake pipe by the low-pressure EGR. As a result, sulfur components in the returned fuel can corrode, for example, an intercooler arranged in the intake pipe.
Moreover, the NOx reduction performance of the LNT rapidly drops when a NOx catalyst of the LNT is deteriorated due to sulfur-poison and heat.
FIG. 11 shows the relationship between the NOx absorption quantity and the NOx reduction rate of the LNT in each of two cases. In the first case, the NOx catalyst is new; in the second case, the NOx catalyst is completely deteriorated. As shown in FIG. 11, in the second case, the NOx reduction rate of the LNT begins to rapidly drop at a much smaller NOx absorption quantity than in the first case.
Furthermore, in the second case, the NOx absorption capacity of the LNT is also lowered and thus a smaller portion of the fuel injected by the fuel injector into the exhaust pipe can be used by the LNT for NOx reduction than in the first case.
FIG. 12 shows the ratio between that portion of the fuel which is used by the LNT for NOx reduction and the remaining portion that passes through the LNT in each of the first and second cases. As shown in FIG. 12, in the second case (i.e., when the NOx catalyst is completely deteriorated), the majority of the fuel injected by the fuel injector into the exhaust pipe passes through the LNT without being used by the LNT for NOx reduction, resulting in an increased concentration of HC in the exhaust gas.